The present invention relates generally to a vehicle transmission control system, and more particularly to a transmission control system for a transmission which is shifted by pressure operated clutches.
It is known to provide an automatic control system for a vehicle powershift transmission having pressure operated clutches controlled by electrohydraulic valves. Some aspects of this technology are described by Goodbar and Testerman in "The Design and Development of a Four Speed Powershift Transmission with Electronic Clutch Pressure Modulation" published in 1986 as SAE Technical Paper No. 861212. One such microprocessor-based control system is further described in U.S. Pat. No. 4,855,913, issued 8 Aug. 1989 to Brekkestran et al. Both of these references disclose that it is known to modulate or gradually increase transmission clutch pressure to achieve a smooth gear shift. With these systems transmission clutch pressure modulation was typically designed to be acceptable under conditions of heavy loading. This results in an abrupt or jerky gear shift under light load conditions. U.S. Pat. No. 4,836,057, issued 6 Jun. 1989 to Asayama et al., also shows a system for controlling transmission speed change clutches. In this system the "pattern" or rate of change of clutch pressure is varied as a function of vehicle weight and a parameter referred to as "speed reduction ratio constant", all for the purpose of limiting "jerk" or speed change shock. However, none of these known systems allow the manner in which transmission clutch pressure is modulated to be varied to compensate for changes in the degree of loading of the drive train. Accordingly, it would be desirable to have a transmission control system which provides smooth shifting under varying conditions of loading.